Ring expansion, by fragmentation

Aldol reactions can be stereoselective 868 Ring expansion by fragmentation 963... 

Cyclopropa[6]naphthalene (3) reacts with nonacarbonyldiiron in dichloromethane at 25 °C resulting in a ring expansion by insertion of a — C(0)Fe(CO)4— fragment to yield 4 in 27% yield, the driving force of the reaction is strain relief of the cyclopropene moiety in 3. ... 

It has been reported that, while ring expansion by ionic fragmentation of a bicyclo[3.3.0]octanol could not be achieved, an alkoxyl radical generated from the octanol resulted in a selective P-cleavage of the fused bond to give a functionalized bridged cyclooctane derivative (Scheme 69). 

The mass spectrum of 2-pyrone shows an abundant molecular ion and a very prominent ion due to loss of CO and formation of the furan radical cation. Loss of CO from 4-pyrone, on the other hand, is almost negligible, and the retro-Diels-Alder fragmentation pathway dominates. In alkyl-substituted 2-pyrones loss of CO is followed by loss of a hydrogen atom from the alkyl substituent and ring expansion of the resultant cation to the very stable pyrylium cation. Similar trends are observed with the benzo analogues of the pyrones, although in some cases both modes of fragmentation are observed. Thus, coumarins. 

Alkoxy radical fragmentation is also involved in ring expansion of 3- and 4-haloalkyl cyclohexanones. The radical formed by halogen atom abstraction adds to the carbonyl group, after which fragmentation to the carboethoxy-stabilized radical... 

The key cyclization in Step B-2 was followed by a sequence of steps that effected a ring expansion via a carbene addition and cyclopropyl halide solvolysis. The products of Steps E and F are interesting in that the tricyclic structures are largely converted to tetracyclic derivatives by intramolecular aldol reactions. The extraneous bond was broken in Step G. First a diol was formed by NaBH4 reduction and this was converted via the lithium alkoxide to a monomesylate. The resulting (3-hydroxy mesylate is capable of a concerted fragmentation, which occurred on treatment with potassium f-butoxide. 

The ring expansion reaction of diaryl cyclopropenones and cyclopropene thiones occurring with pyridinium, sulfonium, and phosphonium enolate betaine 427268-270) is related to 1,3-dipolar cycloadditions. This process results in formation of 2-pyrones 428 by loss of pyridine (or sulfide or phosphine) and insertion of the remaining fragment C=C-0 to the C1(2)/C3 bond of the cyclopropenone ... 

The ring expansion mechanism is of course only a special case of the well-known mechanism by which dioxolan reacts with non-cyclic formals e.g., (I) and CH2-(OMe)2 give (MeOCH2OCH2-)2 in this way. It also accounts in a simple manner for the cleanness of the monomer-polymer equilibrium and for the high yields of cyclic dimer (without any detectable linear fragments) which are obtainable from 1,3-dioxane and 1,3-dioxepan [8]. 

Pattenden and Schulz have reported that treatment of the acetylene derivative 43 with (TMS)3SiH leads, in one pot, to the bicyclic compound 44 in 70% yield (equation 71)". The proposed mechanism involves (TMS)3Si radical addition to the triple bond to form a vinyl radical followed by a remarkable cascade of radical cyclization-fragmentation-transannulation-ring expansion and termination via ejection of the (TMS Si radical to afford the bicyclic product. 

Similarly, exposure of stannyl lactol to PhI(OAc)2 4 led to oxidative ring expansion yielding the trans unsaturated lactone stereoselectively, presumably via 1,4-fragmentation induced by reductive elimination of iodobenzene [Eq. (46)] [83]. 

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